Prevent user to order the same product twice in an a single purchase order

lacagette_addons/lacagette_purchase/models/purchase_order.py

 # -*- coding: utf-8 -*-
 
 from openerp import api, models, fields
-
+from openerp.tools.translate import _
+from openerp.exceptions import UserError
 
 
 class PurchaseOrder(models.Model):
     _inherit = 'purchase.order'
 
+    def verify_product_line_unicity(self):
+        pdts = {}
+        for order in self:
+            for line in order.order_line:
+                if not line.product_id.id in pdts:
+                    pdts[line.product_id.id] = 0
+                pdts[line.product_id.id] += 1
+        pids = []
+        for pid, nb in pdts.items():
+            if nb > 1:
+                pids.append(pid)
+        if len(pids) > 0:
+            products = self.env['product.product'].search([('id', 'in', pids)])
+            message = ''
+            for p in products:
+                message += p.display_name + ' (' + str(p.product_tmpl_id.id) + ')'
+                message += ' --> ' + str(pdts[p.id]) + " lines\n"
+            raise UserError(_(message))
+    
     @api.multi
     def write(self, vals):
         res = super(PurchaseOrder, self).write(vals)
         if 'state' in vals and vals['state'] == 'purchase':
             import requests
+            # First of all, verify that no product has been added twice or more
+            # an error is raised if a product appears more than once
+            self.verify_product_line_unicity()
             conf = self.env['ir.config_parameter']
             export_url = str(conf.get_param('cagette_purchase.orders_proxy_url')).strip()
             export_url += '/export/' + str(self.id)